Changes MergeRequestApi:

- added methods "getDiffs()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/diffs

- add methods "getDiffVersions()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions

- add method "getDiffVersion()" and "getOptionalDiffVersion()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id

- deprecate "getMergeRequestDiffs()" and "getMergeRequestDiff()"

Fixes #993

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>